Energy efficient compact fluorescent lamps are very much in demand today. Environmentally conscious people are turning to these lamps to cut down significantly on the usage of electricity without sacrificing lumens. The decrease in the consumption of electric power translates to an appreciable lowering in the emissions of acid rain precursors and green house gases from power plants.
Presently, most compact fluorescent lamps are made using water base coating technology which suffers from several drawbacks. An electrostatic coating method would eliminate suspension degradation due to the characteristic long term holdover in water base systems. Loss of phosphor due to redispersion problems with drying chamber belt scrapings would also be done away with. In addition, an electrostatic coating process would make shrinkage, due to lack of adequate insolubility of the first coat in conventional two coat lamp systems, a thing of the past. Also, problems due to progressive flocculation of Aluminum Oxide C in first coat suspensions would be solved. Furthermore, an electrostatic coating process would realize powder weight savings since it needs small particle size phosphor. Use of small particle size phosphors in water base coatings is difficult due to the high aqueous reactivity of such materials and the imperfections in protective coatings deposited on such powders. Another advantage of an electrostatic coating process lies in the elimination of the need for a dedicated brushing machine after bakeout, since electrostatically coated lamps can be brushed easily before bakeout by a unit mounted in the coating station. This yields savings in maintenance since there is one less machine, the brusher, to maintain. Finally, an electrostatic coating unit needs a smaller footprint than a comparable water base coating system.
U.S. Pat. No. 5,032,420 discusses a method of manufacturing Cd free yellow incandescent bug lights. The emphasis is on the powder which is used for the coating process: ZrSiO.sub.4 /Pr mixed with 10% amorphous silica. There is no mention of the use of a polymer in this description. U.S. Pat. No. 4,914,723 discloses a standard fluorescent lamp discharge tube with an electrostatically coated envelope. The phosphor is pretreated prior to the electrostatic coating step, by coating the powder with a fatty acid (like palmitic, oleic etc.) or a NH.sub.4, Al or alkaline earth salt and finely divided alumina. The relative composition of the mix is 100 parts by weight phosphor, 0.01 to 3 parts by weight fatty acid and 0.05 to 5 parts by weight fine particle size alumina. This work does not use any polymeric material in the phosphor pretreatment step.
U.S. Pat. No. 4,689,172 teaches a process for the surface treatment of phosphors prior to electrostatic coating of the material on low pressure mercury vapor discharge lamps. The treated material can preferably be used for coating the envelope of compact fluorescent lamps as well as standard size fluorescent lamps. In the course of suspension, the luminescent materials are treated with acids the anhydrides of which are susceptible to the formation of low melting point glasses. Aliphatic amines are subsequently adsorbed onto the acid treated particle surfaces . According to the inventors, the double acid-alkylamine layer makes the particles hydrophobic and more easily sprayable and dischargeable. In the course of heat treatment, the amine is removed without leaving any C contaminants. The glass forming oxide liberated during heat treatment adheres the particles together and to the glass. Examples of glass forming acids are boric and phosphoric acid while examples of amines are n-butyl and n-octyl. Polymers do not figure in the surface treatment taught in this patent.
U.S. Pat. No. 4,610,217 relates to an apparatus for the electrostatic coating of the insides of different kinds of bulbous lamp envelopes. Means of producing a homogeneous smoke of the powder are discussed. No special pretreatment of the phosphor is discussed. U.S. Pat. No. 4,597,984 presents a method and apparatus for coating fluorescent lamp tubes. In particular, this work focuses on the U lamp and the twin tube configuration lamp (GE Mod-U-Line). This invention teaches the use of dual electrodes: one is stationed outside the U tube and a pair are located inside the two legs. The electrodes move in sequence, the movement being controlled by a motor mechanism. The glass tube is maintained between 150.degree. C. and 500.degree. C. to render it conductive. After the deposition step, the phosphor coating is humidified by blowing saturated air over the layers resulting in a decrease in voidage and a more compact coating. Humidification is followed by a low temperature bakeout, 475.degree. to 600.degree. C., which removes any moisture from the coating and bonds the phosphor particles to the glass and to themselves. As set forth in column 6, lines 31 to 36, "A lower lehring temperature may be employed following the electrostatic deposition according to the present invention than is employed in prior art slurry deposition, because no organic binder containing carbon materials is used to initially bond the phosphor coating to the glass".
U.S. Pat. No. 4,081,714 teaches pretreatment of phosphors for electrostatic coating of standard size fluorescent lamps. In particular, 100 parts by weight phosphor are mixed with 0.01 to 1 part by weight stearic, palmitic acid or salts thereof, 0.1 to 3 parts by weight Ca or Sr nitrate for good adhesion and 0.05 to 5 parts by weight fine alumina for flowability. Polymers are not used in this invention. JP 7639509 and JP 7638357 discuss electrostatic coating of standard fluorescent lamps wherein phosphor powder is mixed with 0.1 to 8 weight % calcium pyrophosphate or low melting point glass frit and charged at 18-22 kV. DD 156,497 teaches the coating of phosphor powders onto lamp envelope walls by mixing phosphor with diammonium hydrogen phosphate, coating the mixture electrostatically and heating. Neither the Japanese nor the East German patents discuss the use of a polymeric material for the surface treatment of the phosphor.